Keith Hunter of the Chemistry Department, University of Otago, responded.

Firstly, to understand this question it is important to remember that
electricity is conducted by electrical charges. In a wire, and inside
the circuits of a TV, radio or computer, electricity is conducted
mainly by electrons which are negatively charged.

Water that contains a dissolved salt (e.g. river and sea waters)
conducts electricity mainly through the ions of the salt. Salts are
compounds that when they dissolve, break up into species called ions
that are positively and negatively charged. For example, table salt
produces positive sodium ions and negative chloride ions. The negative ions
move towards the positive electrode and the positive ions move
in the opposite direction. The conducting ability of a salt solution
depends directly on how much salt is present - the more salt, the
greater its ability to conduct electricity. Because salt solutions
conduct electricity well, chemists call them electrolyte solutions.

Even absolutely pure water contains some ions - these are called
hydrogen ions (positive) and hydroxide ions (negative). They are
produced from the breakdown of a very small number of water molecules.

In the real world, even the purest water also contains ions from
impurities that are very difficult to keep out of the water. For
example, carbon dioxide gas will dissolve from the air to produce
bicarbonate ions.